Heat 2 tablespoons of olive oil in a large ovenproof skillet over medium heat. Add thinly sliced onions and sauté for 5-7 minutes until soft and translucent.

Sprinkle red pepper flakes, salt, and pepper to taste. Pour in ½ cup of dry white wine and juice from half a lemon. Simmer for 2-3 minutes.

Carefully place cod fillets atop the mixture. Season with salt and pepper. Let the fish soak in the sauce for a minute.

In a bowl, combine 1 cup of panko breadcrumbs, 2 tablespoons of melted butter, 1 tablespoon of olive oil, and remaining lemon juice. Mix until breadcrumbs are coated, and fold in chopped parsley.

Preheat oven to 400°F (200°C). Spread panko mixture over cod fillets and bake for 20 minutes.

Remove from the oven and let rest for a few minutes. Garnish with parsley and lemon slices before serving.